A. Field of the Invention
The present invention relates to a piezoelectric gyro that vibrates in a direction of detected vibration orthogonal to a direction of driven vibration in accordance with an angular velocity added when the gyro is excited in the direction of driven vibration, and to a method of driving such a piezoelectric gyro.
B. Description of the Related Art
Motion of a substance may be mathematically divided into translational motion and rotational motion. A piezoelectric gyro that can detect only rotational motion independently of translational motion is known. Such a piezoelectric gyro is constructed to serve as, for instance, a tuning-fork piezoelectric element having two arms extending in parallel with each other from a base. Further, drive electrodes are attached to one of the arms and detecting electrodes to the other arm. The drive electrodes are positioned so that the arm vibrates within a plane when a drive signal is supplied to the drive electrodes. If an angular velocity derived from rotational motion is added when the arm is in a driven vibration mode after the drive signal has been supplied to the drive electrodes, a Coriolis force acts to cause the arm to vibrate in a direction orthogonal to a direction of driven vibration, i.e., perpendicularly to the plane. If the detecting electrodes are arranged so that such vibration perpendicular to the plane can be detected, the added angular velocity can be detected based on an output signal from these detecting electrodes. Such a piezoelectric gyro comes in the following types as typical ones: a column type that has a circular, triangle, or rectangular cross-section; a tuning fork type that has three or more arms; and an H type that has arms extending from opposite ends of a base. These piezoelectric gyros are widely used, e.g., for detecting unintentional movement of the hands in taking a picture with a video camera, detecting a yaw rate in driving an automobile, and detecting the position of a mobile body in a navigation system or the like.
In such a piezoelectric gyro, by making a resonant frequency in a driven vibration mode closer to a resonant frequency in a detected vibration mode, detection sensitivity can be improved. However, it is difficult to make the resonant frequency in the driven vibration mode exactly equal to the resonant frequency in the detected vibration mode due to errors derived from the coupling of the respective vibration modes and the like. It has heretofore been common to drive the gyro at the resonant frequency in the driven vibration mode which is out of phase with the resonant frequency in the detected vibration mode. Further, to stabilize the excited state at such resonant frequency in the driven vibration mode, it is feasible to set the gyro into self-excited oscillation utilizing driven resonance.
In the aforementioned gyro, when the gyro is driven at the resonant frequency in the driven vibration mode, there is always a strong possibility that adequate detection sensitivity will not be obtained. Further, in some piezoelectric vibrators, the resonant frequency in each vibration mode changes with changing temperature, and this requires that a detected signal be subjected to temperature correction. For example, when a tuning fork-type or H-type piezoelectric vibrator is constructed using a tantalic acid lithium single crystal (LiTaO.sub.3), a resonant frequency for vibration within a plane (fx mode) exhibits satisfactory stability with respect to temperature, while a resonant frequency for vibration perpendicular to the plane (fz mode) is changed greatly by temperature. When the gyro is excited at an fx-mode resonant frequency using such a vibrator made of a tantalic acid lithium single crystal while specifying the fx mode as the driven vibration mode and the fz mode as the detected vibration mode, the detection sensitivity changes greatly with changing environment, and this requires that temperature correction be made.